The present invention relates to wireless base-station systems which are provided in mobile communication systems employing Code Division Multiple Access technology and which perform wireless communication with mobile terminal devices. More specifically, the present invention relates to a wireless base-station system which includes a base-station main apparatus and local wireless apparatuses.
In mobile communication systems employing Code Division Multiple Access (hereinafter called CDMA) technology, since a range where radio waves from a wireless base-station system reach is a communication area, mobile terminal devices cannot be used in dead zones which are beyond the reach of radio waves from a wireless base-station system, that is, in tunnels and underground areas.
In mobile communication systems of recent years, with the widespread use of mobile terminal devices such as mobile telephones, since a wireless base-station system needs to process data for a large number of users, the apparatus configuration is increasingly becoming complicated and large. Thus, a configuration in which the wireless base-station system is separated into a base-station main apparatus and local wireless apparatuses is known. In the configuration, the base-station main apparatus performs processing on a baseband signal for each mobile terminal device and the local wireless apparatuses amplify power for an RF (radio frequency) signal, modulate and demodulate the RF signal, and have antenna devices.
The local wireless apparatus is relatively small compared to the base-station main apparatus and thus can be installed in a subway station premise or underground arcade. Thus, such a configuration is also effective to eliminate the dead zones.
In general, since a communication area managed by a wireless base-station system is divided into a plurality of service areas 106-1 to 106-m (m is a positive integer), as shown in FIG. 1, local wireless apparatuses 103, which have a similar configuration, are installed in the respective service areas 106-1 to 106-m which are away from a base-station main apparatus 101 and the local wireless apparatuses 103 are connected to the base-station main apparatus 101 through respective optical transmission paths (e.g., optical fibers) 104.
For the purpose of expanding a service area for a wireless base-station system, Japanese Unexamined Laid-Open Publication (JP-A) No. 11-284639 discloses a configuration in which a large number of slave repeaters for performing wireless communication with mobile terminal devices and a master repeater relays a signal transmitted/received between a base-station main apparatus and each slave repeater. The configuration of such a wireless base-station system including a master repeater and multiple slave repeaters is shown in FIG. 2.
FIG. 2 is a block diagram showing the configuration of a known wireless base-station system including a master repeater and multiple slave repeaters.
The known wireless base-station system shown in FIG. 2 includes a base-station main apparatus 201, a master repeater 202 for transmitting/receiving an RF signal to/from the base-station main apparatus 201, and multiple slave repeaters 203 connected with the master repeater 202 via optical transmission paths. Only one slave repeater 203 is shown in FIG. 2.
The base-station main apparatus 201 generates an RF signal for communication with each mobile terminal device and transmits the signal to the master repeater 202. On the basis of Patent Application Publication he RF signal received from the slave repeater 203 via the master repeater 202, the base-station main apparatus 201 reproduces a signal for each mobile terminal device and transmits the signal to a high-order apparatus (not shown), such as a known exchange.
The master repeater 202 has an optical/electrical (O/E) converter 205 to convert an RF signal received from the base-station main apparatus 201 into an optical signal and to convert an optical signal received from each slave repeater 203 into an RF signal.
The slave repeater 203 includes an antenna device 208, an O/E converter 206 for converting an optical signal received from the master repeater 202 into an RF signal and converting an RF signal to be transmitted to the master repeater 202 into an optical signal, and a duplexer (DUP) 207 for switching transmission channels between the O/E converter 206 and the antenna device 208 in response to the transmission/reception of an RF signal. The optical signal transmitted/received between the master repeater 202 and the slave repeater 203 is generated by performing analog modulation on light based on an RF signal.
With this configuration, when an RF signal for each mobile terminal device is transmitted from the base-station main apparatus 201 to the master repeater 202, the RF signal is converted by the O/E converter 205 into an optical signal, which is then distributed to the slave repeaters 203. In the slave repeater 203, the O/E converter 260 converts the optical signal received from the master repeater 202 into an RF signal and the duplexer 207 radiates the RF signal into the service area 204 via the antenna device 208.
On the other hand, an RF signal received by the antenna device 208 of the slave repeater 203 is converted by the O/E converter 206 via the duplexer 207 into an optical signal, which is then transmitted to the master repeater 202 via the optical transmission path. In the master repeater 202, the optical signal received from the slave repeater 203 is converted by the O/E converter 205 into an RF signal, which is then combined with RF signals received from the respective slave repeaters and the resulting signal is transmitted to the base-station main apparatus 201.
Since the wireless base-station system shown in FIG. 1 has a configuration in which the local wireless apparatus is installed for each service area having a predetermined area, there are some problems. For example, at a place where a wide service area is required even though the subscriber density is low, for example, in a mountain region or sparsely-populated region, or at a place where a large number of relatively small service areas are required because of the intricate topography, for example, in a building or in an region in which large-scale buildings are close together, an area which can be covered is limited due to shortage of the number of areas. As a result, a “dead zone” is created, even if the subscriber capacity can be ensured.
With the configuration (shown in FIG. 2) in which the O/E converters 205 and 206 are used to transmit/receive an RF signal between the master repeater 202 and the slave repeater 203, the NF (noise figure) is severely degraded by the O/E converters 205 and 206. Thus, there is a problem in which the reception sensitivity of the slave repeater 203 is reduced and thus the service area cannot be expanded.